Re: [gentoo-user] btrfs: subvol without compression

[Marc Joliet]

I don't know if you eventually found an answer yourself, but since nobody else
has replied, here's what I found:

Am Tue, 17 Jun 2014 13:04:08 +0200
schrieb "Stefan G. Weichinger" <lists@×××××.at>:

>
> ... I am quite happy now with the performance of that new server I am
> preparing.
>
> See thread "Intel(R) C600 SAS Controller" for that story:
>
> https://www.mail-archive.com/gentoo-user@l.g.o/msg146119.html
>
> Right now I get quite good results when doing backups of the 2 existing
> VMs (which have their virtio-disks on LVM-LVs on the host) ... up to
> 200MB/s ... I can show for reference, if someone is interested.
>
> I think that is around the possible maximum.
>
> --->
>
> The issue I want to share with you is related to a btrfs subvol I have here.
>
> Block device sda builds the btrfs-pool containing the root-fs:
>
> # btrfs fi show
> Label: ROOT  uuid: 9133c469-df1e-45f5-a09f-d1b9c75c69da
> 	Total devices 1 FS bytes used 29.47GiB
> 	devid    1 size 500.00GiB used 278.04GiB path /dev/sda
>
> Btrfs v3.12
>
> These are the subvolumes (I could/should rm some, but it doesn't matter
> for this issue, afaik):
>
> # btrfs su list /
> ID 257 gen 4282 top level 5 path __active
> ID 258 gen 4874 top level 5 path __active/root
> ID 266 gen 4772 top level 258 path images
> ID 267 gen 838 top level 258 path images/otrs
> ID 289 gen 4285 top level 258 path images/windows
> ID 538 gen 4874 top level 5 path __active/virt-backup
>
>
> fstab has:
>
> # grep btrfs /etc/fstab
> LABEL=ROOT	/		btrfs	defaults,noatime,compress=lzo	0 0
> LABEL=ROOT	/mnt/virt-backup	btrfs	compress=no,noatime,subvolid=538 0 0
>
>
> ... so I want to mount subvolid 538 with disabled compression (to speed
> up backups as the files written to it are compressed on the fly via pigz
> already).

So far nothing in particular stands out, that is, it looks like everything is
correct.

> But after booting I get that dir mounted with compress=lzo (which is
> default).
>
> # mount | grep btrfs
> /dev/sda on / type btrfs (rw,noatime,compress=lzo,space_cache)
> /dev/sda on /mnt/virt-backup type btrfs
> (rw,noatime,compress=lzo,space_cache)
>
> remounting works, though:
>
> booze ~ # mount -o remount,compress=no /mnt/virt-backup/
>
> booze ~ # mount | grep btrfs
> /dev/sda on / type btrfs (rw,noatime,space_cache)
> /dev/sda on /mnt/virt-backup type btrfs (rw,noatime,space_cache)
>
> BUT it remounts / without compression as well ... !
>
> Is it a bug? A mistake or misunderstanding?

From my own google search, at least up to 2011 per-subvolume compression
settings were not possible.  Then, after subsequently searching on the btrfs
wiki for a while, I finally found an answer: no.  See this FAQ entry:

https://btrfs.wiki.kernel.org/index.php/FAQ#Can_I_mount_subvolumes_with_different_mount_options.3F

[...]

HTH
-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup